In conventional heat exchangers structural strength for pressure containment of the fluids dominates design considerations. Metal tube-in-tube and tube-in-shell exchangers are examples of such designs.
The need has arisen for a low cost, low pressure heat exchanger that is not susceptible to the corrosion and scaling which typifies similar devices constructed of metal.
The heat exchanger of the present invention reduces operating pressures to the level that pressure containment does not dictate the shape or form of the mechanical configuration. This design freedom allows the usage of previously unacceptable materials of construction such as structurally strong plastic extrusions and thin plastic membranes for heat transfer surfaces. The use of low cost materials and the low pressure design of the system also allow the use of more complex flow patterns to achieve novel and heretofor unexpected results from coolers or evaporators using direct contact between liquid and gas.
The reduction in operating pressure is achieved with two significant departures from the prior art. First, heat transfer starts with the establishment of a uniform falling liquid film on a membrane type of heat exchange surface. The falling film is established with low pressure nozzles, (e.g. open pipes), which require only 1-2 p.s.i.g. operating pressure in the distribution manifolds, and even less pressure in the gravity fed fluid collection manifolds. Secondly, air plenums coupled with pressure relief ports and ducts are used to reduce the gauge pressure across the heat transfer surface to nearly zero.
In the apparatus of the present invention the uniform falling film of highly concentrated solutions, established with very low pressure nozzles, avoids air entrainment due to splashing. Entrainment due to surface stripping of water droplets is avoided by controlling the relative velocity between the air draft and the falling film.
With this background it is seen that the primary object of the invention is to provide a low pressure heat exchanger apparatus having a large degree of design flexibility to enhance the operating effectiveness of direct liquid-gas contact and liquid to liquid heat transfer.
A further object of the invention is to provide a heat exchange device which has low area unit cost for fabrication and operation.
Another object is to provide a heat exchanger which can simultaneously or alternatively perform liquid to liquid heat exchange, liquid to gas heat exchange and liquid to gas mass transfer.